Selective isolation of polycyclic aromatic hydrocarbons by self-assembly of a tunable N→B clathrate

Article abstract of DOI:10.1021/acs.cgd.5b00219

The combination of one dipyridyl linker [1,2-di(4-pyridyl)ethylene (DPE), 1,2-di(4-pyridyl)ethane (DPEt), or 4,4′-azopyridine (DPA)] with two molecules of arylboronate ester 1 produced dinuclear Lewis-type N→B adducts that can act as acyclic host for polycyclic aromatic hydrocarbons (PAHs) in the solid state. Nine crystalline solids of composition PAH@adduct (i.e., one PAH per adduct) were obtained from solutions containing a single PAH. On the basis of the single-crystal X-ray diffraction analysis of the compound ANT@A1 (ANT = anthracene; A1 = adduct being composed of DPE and two boronate esters 1), the PAH inclusion selectivity is related to a size-fitting adaptation to an octaedral-shaped pocket assembled by CH-π interactions between fragments of the diamine and the arylboronate ester 1. The resulting reversible organic clathrates can perform catch and release cycles of PAHs such as pyrene and can sequester selectively PAHs from mixtures in solution.

Full text of DOI:10.1021/acs.cgd.5b00219

Communication
pubs.acs.org/crystal
Selective Isolation of Polycyclic Aromatic Hydrocarbons by Self-
Assembly of a Tunable N→B Clathrate
Angel D. Herrera-Espana,† Gonzalo Campillo-Alvarado,† Perla Roman-Bravo,† Dea Herrera-Ruiz,‡
́
̃
Herbert Hopfl,† and Hugo Morales-Rojas*,†
̈
†Centro de Investigaciones Químicas, and Facultad de Farmacia, Universidad Auton
1001, C.P. 62209 Cuernavaca, Morelos, Mexico
́
́
oma del Estado de Morelos, Av. Universidad
‡
S
* Supporting Information
ABSTRACT: The combination of one dipyridyl linker [1,2-di(4-pyridyl)ethylene (DPE), 1,2-di(4-pyridyl)ethane (DPEt), or
4,4′-azopyridine (DPA)] with two molecules of arylboronate ester 1 produced dinuclear Lewis-type N→B adducts that can act as
acyclic host for polycyclic aromatic hydrocarbons (PAHs) in the solid state. Nine crystalline solids of composition PAH@adduct
(i.e., one PAH per adduct) were obtained from solutions containing a single PAH. On the basis of the single-crystal X-ray
diffraction analysis of the compound ANT@A1 (ANT = anthracene; A1 = adduct being composed of DPE and two boronate
esters 1), the PAH inclusion selectivity is related to a size-fitting adaptation to an octaedral-shaped pocket assembled by CH-π
interactions between fragments of the diamine and the arylboronate ester 1. The resulting reversible organic clathrates can
perform “catch and release” cycles of PAHs such as pyrene and can sequester selectively PAHs from mixtures in solution.
he molecular recognition of polycyclic aromatic hydro-
carbons (PAHs) is a challenging issue given the variety of
recognized as a useful strategy for the generation of complex
systems, and given its thermodynamic stability (−11 to −38
kJmol−1)6,7 and kinetic lability in polar coordinating solvents,
the central N→B binding motif can be compared to a typical
noncovalent interaction. The potential of this approach is well
exemplified by the prismatic organic cages assembled via N→B
bonds that were able to encapsulate triphenylene or coronene
in the solid state.6c These previous reports have also evidenced
that simple arylmonoboronate esters normally form 2:1 Lewis-
type adducts in the presence of diamines such as 4,4′-bipyridine
(BiPY) or 1,2-di(4-pyridyl)ethylene (DPE).7 As a consequence,
the dipyridyl aromatic moieties become electron-deficient, and
because of being embedded in a tweezer-type arrangement,
they might become suitable to recognize electron-rich species.
Since a systematic search about the prospects of these discrete
species to act as acyclic molecular host for PAHs is still missing,
this turned on our interest in exploring the supramolecular
chemistry between discrete N→B adducts and electron-rich
PAHs.
T
sizes, shapes, and properties of these widespread materials.1
PAHs are among the most intensively studied pollutants in the
environmental analysis of air, water, food, and solid-waste
samples. Methods for quantitative separation and detection of
PAHs often rely on traditional chromatographic and solvent-
extraction protocols.2 A well-established supramolecular
approach to bind PAHs in solution consists of the use of
polycationic macrocycles and cages derived from extended
viologens, which are assembled covalently3 or through metal
coordination.4 The association constants of these host−guest
complexes in water and polar solvents range between 101 and
106 M−1, and a significant number of X-ray crystal structures of
such complexes has been reported. Yet these elegant supra-
molecular receptors were made in multistep synthetic
procedures3 or involved costly transition metals.4 Thus, it is
still a demanding task to develop simple methods to selectively
detect PAHs, and also to be able to recover them quantitatively
as a single (or multiple) PAH from mixtures in solution.1,2
In the past decade, various research groups have
comprehensively explored the interaction of aromatic amines
with boronate esters and boroxines to provide a broad variety
of discrete and polymeric materials, such as macrocycles,5
nanostructures,6 coordination polymers,7 and gels.8 Hence,
supramolecular assembly based on dative N→B interactions is
Herein, we report the first example of a family of discrete
tweezer-type N→B adducts that starting from three or four
molecular components assemble into solid-state clathrates9
Received: February 12, 2015
Published: February 26, 2015
© 2015 American Chemical Society
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Scheme 1. Molecular Components Used to Assemble N→B Adducts Containing PAHs as Crystalline Solids
with inclusion of PAHs. Initial condensation of phenylboronic
acid (PBA) and 2,3-dihydroxynaphthalene (DHN) in acetoni-
trile gave boronate ester 1 in good yields (Scheme 1). Upon
stirring a chloroform solution containing 1 and DPE in a 2:1
molar concentration ratio for 1 h at RT, a solid precipitated that
could be isolated by filtration (A1, 91%). A1 presented a
powder X-ray diffraction (PXRD) pattern different from that of
the starting materials (Figure 1a−c) and the NMR spectro-
according to the thermogravimetric analysis (TGA) resulted to
be the acetone solvate of A1 (Figure S2, SI). Single-crystals
suitable for X-ray diffraction analysis could be grown from
acetone at T < 6 °C that evidenced the suggested molecular
composition and the presence of dative N→B bonds (Figure 2
and Figure S3, SI). In the crystal lattice, the boron atoms are
four-coordinated with a tetrahedral character (THC)10 of
79.6% and N→B bond lengths of 1.6321(18) Å. Two
molecules of acetone are included per A1 adduct, which are
confined in pockets generated by the phenyl and naphthoyl
substituents of the boronate ester and bound through
CHacetone···π and CHpyridyl···Oacetone interactions (Figure 2c,d).
After this promising result, a systematic screening was
performed in order to evaluate if PAHs can be also included
(PAHs used: naphthalene (NAP), fluorene (FLR), phenan-
threne (PHE), anthracene (ANT), pyrene (PYR), triphenylene
(TRP), benzo(a)anthracene (BaA), and perylene (PER); see
structures in Scheme 1). Stirring of 1, DPE and the
corresponding PAH in a 2:1:1 molar ratio in chloroform
solution at RT for 1 h afforded new crystalline solids in the
presence of NAP, FLR, PHE, ANT, and PYR (Table 1),
respectively, that were isolated by simple filtration of the
reaction product. Each of these solids showed an entirely
different diffraction pattern when compared to A1 or the
starting materials (see Figure 1e−i). Notably, the diffraction
patterns are quite similar and display a characteristic peak
around 2θ = 7.5°. As representative example, ANT@A1 was
assembled also in 48% yield in CHCl3 from a four-component
mixture of PBA, DHN, DPE, and ANT in a 2:2:1:1 ratio. The
lability of both the N→B and B−O bonds in aqueous
methanolic solutions enables to dissociate all of these solids
again into the starting components, as shown by HPLC
analysis, which allowed us to identify and to quantify
individually PBA (as the methyl ester), DHN, DPE, and the
corresponding PAH (see Table S1 and Figure S4, SI).11
Composition of these solids was found having the same
stoichiometric ratio PBA/DHN/DPE/PAH (2:2:1:1), indicat-
ing a general composition of A1/PAH (1:1).
Figure 1. PXRD patterns from top to bottom: (a) 1; (b) DPE; (c) A1;
(d) (acetone)2@A1; (e) NAP@A1; (f) FLR@A1; (g) PHE@A1; (h)
ANT @A1; (i) PYR@A1; (j) toluene@A1.
scopic analysis in DMSO-d6 showed signals for both 1 and DPE
in the expected 2:1 ratio (Figure S1, Supporting Information
(SI)). Adduct A1 could also be isolated by performing the
reaction in refluxing acetone starting from a multicomponent
mixture of PBA, DHN, and DPE in a 2:2:1 ratio, conditions
that point out that in situ formation of the boronate ester 1 is
achieved easily.
By lowering temperature of the above-described mixture in
acetone to 6 °C, a novel crystalline solid precipitated, that
Compound ANT@A1 crystallized as orange blocks that
displayed fluorescence under UV-light (see Figure S5a, SI) and
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Figure 2. X-ray structure of (acetone)2@A1. (a) Assymetric unit; (b) view along the ac plane that shows the relative orientation of the acetone guest
molecules; (c) CHacetone···π interactions; (d) CHpyridyl···Oacetone and CHpyridyl···Oboronate interactions. Distances are in angstroms.
Table 1. Results of the Screening Experiments for the Formation of Clathrate Compounds from Combinations of 1 with
d
Dipyridyl Linkers
a
Structural parameters for PAHs molecules are shown below the three letter code corresponding to the maximum dimensions in angstroms for the
b
length (L), breadth (B), and L/B ratio according to the Polycyclic Hydrocarbon Structure Index NIST special publication 922. Structural
parameters L, B, and L/B ratio for dipyridyl molecules are shown and were calculated using the Spartan’10 software package. Solubility of the solids
c
in CHCl3 at 25 °C; value is given in mM concentration units. All solids remained stable during the elapsed time of these experiments (3 h) as
d
indicated by the corresponding PXRD analyses. Green check mark: A new crystalline solid precipitated out from solution. Color under ambient
light and fluorescence (F) under UV light exposure is also indicated. Red X-marks: No precipitate was observed or just the adduct A1 or A2 was
recovered.
was further examined by single-crystal X-ray diffraction
analysis.12 The crystal lattice is essentially built up by
interactions between the A1 adducts, in which the fragments
from 1 and DPE play different roles. The DPE fragments
embed the ANT molecules in a sandwich-like manner into
infinite π-π stacked pillars along the a axis (Figure 3a,b, green/
blue CPK) with centroid···centroid distances ranging from 3.52
to 3.87 Å.
The boronate ester fragments from neighboring A1 adducts
(Figure 3a,b red stick) complete the ensemble by joining the
DPE fragments via three CH···π interactions, giving octahedral-
shaped pockets containing the ANT guest (Figure 3d).
Interestingly, there are no appreciable intermolecular inter-
actions with the guest molecules other than van der Waals
contacts. This organization indicates a cooperative process
between the adduct components to build up a pocket that
comfortably accepts ANT (L = 11.65 Å, B = 7.43 Å, L/B =
1.57). The same framework must be present in the other
inclusion compounds given the similarity of the PXRD
patterns, and therefore these pockets are endowed with some
adaptability, which allows for the inclusion of other PAHs
within a size range of L = 9.20−11.75 Å, B = 7.43−9.28 Å and
L/B = 1.24−1.52 (i.e., NAP, FLR, PYR, and PHE). Longer or
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Figure 3. X-ray structure of ANT@A1. Panels (a) and (b) display different views of the A1 ensemble that includes ANT. (c) View of the bc plane
showing the relative orientation of the ANT guest molecules. (d) Boron atoms forming an octahedral-shaped pocket containing ANT.
broader PAHs, such as BaA, PER, or TRP (see Table 1), did
not afford inclusion compounds under these conditions.
To further test of these structural features, screening
experiments with PHAs were also performed with mixtures of
1 and 1,2-di(4-pyridyl)ethane (DPEt), 4,4′-azopyridine (DPA)
or 4,4′-bipyridine (BiPy). In the absence of PAHs, the 2:1
adduct A2 (1:DPEt) precipitated as an inclusion complex with
DHN, whereas the adduct A3 (1:DPA) was obtained as a
homogeneous solid (see Figures S5b, S5c, S7, and S8, SI).
Solutions of 1 with BiPy in CHCl3 remained without
precipitates, but in the presence of a single PAH just the 2:1
adduct 1:BiPy was recovered. However, from solutions of 1,
DPEt, and a single PAH, three cocrystalline solids having the
composition A2/PAH (1:1) were formed with ANT, PHE, and
are summarized in Table S2. Rather than a single PAH, the
PYR. With DPA, only one solid precipitated (with PYR) with
three multicomponent solids collected a varying distribution of
the composition A3/PYR (1:1) (see Table 1, Figures S7 and
PAHs. PYR and PHE were present in all three systems and
S8, SI).
were also the most abundant guests, comprising 60−70% of the
PAHs removed from the solution mixture. Unusually, PAHs
Since the diamines DPE, DPEt, and DPA hold a similar
length (see Table 1), it seems that the nature of the diamine
that formerly did not precipitate from a single component
linker plays an essential role and changes the selectivity of the
solution (i.e., TRP, BaA, and PER) were also found in the solid
crystallization-induced formation of these clathrates in chloro-
mixture although in minor quantities. Of the adducts, A1 was
the most promiscuous, incorporating all PAHs with quantities
in the following order PYR > PHE > ANT ≫ PER > FLR ∼
NAP > BaA >TRP. A3 was the most selective clathrate, leaving
form solutions, indicating that electronic complementarity is
also required, particularly, for the smaller PAH guests such as
NAP or FLR.
Screening experiments using other boronate esters (2−5),
DPE and a single PAH produced mostly only the
corresponding 2:1 adducts, without inclusion of the PAH,
emphasizing the importance of the right orientation, size, and
the availability of C-H atoms to arrange the recognition pocket.
Finally, a couple of experiments were carried out to test the
feasibility of applications for the 1-DPE/DPEt/DPA systems.
First, an equimolar solution containing all eight PAHs included
in this study (0.2 mmol each) was stirred with 1 (0.4 mmol)
and 1 equiv of the dipyridyl linker (0.2 mmol of DPE, DPEt, or
DPA) in chloroform. Solids precipitated after 1 h and showed a
PXRD pattern that resemble those corresponding to the solids
isolated from the experiments with single PAHs (Figure S9, SI).
The HPLC traces of the PAHs released from these solids in
aqueous methanol are depicted in Figure S10a,b, and the data
out NAP, ANT, and BaA, and increasing twice the relative
amount of PYR incorporated in the lattice. Therefore, the
distribution of PAHs in these solids is not driven solely by their
intrinsic solubility with values in the mM range (Table 1). For
instance, PYR@A1 is the less soluble inclusion compound, and
therefore, it is expected to dominate in the mixture. However,
NAP is present 7-fold less than PYR in 8PAHs@A1 or
8PAHs@A, albeit NAP@A1 is slightly more soluble (2.1 mM
vs 2.6 mM, PYR vs NAP). In the same context, PHEN and
ANT were included in these solids at a constant 3:2 ratio in
spite of their similar solubility.
In a second experiment, solid toluene@A113 (100 mg, 0.130
mmol) was added to a PYR solution in CHCl3 (2 mL, 68 mM)
and its color changed immediately from yellow to orange. After
this mixture was stirred for 1 h, the solid was removed by
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N. A.; Stern, C. L.; Sarjeant, A. A.; Stoddart, J. F. Angew. Chem., Int. Ed.
2015, 54, 456.
(4) (a) Peinador, C.; Pía, E.; Blanco, V.; García, M. D.; Quintela, J.
filtration giving 98 mg of the compound PYR@A1, thus
removing 88% of the PYR present in the initial solution (Figure
S11). This latter solid (98 mg) was suspended in 3 mL of
toluene and heated up to 100 °C for 1 h. The insoluble material
was removed by filtration giving 68 mg of the starting solvate
toluene@A1 with no trace of PYR@A1 according to the PXRD
analysis (Figure S11, SI). Thus, as determined from the toluene
filtrate by UV−vis spectroscopy, PYR was fully released back
into solution.
In conclusion, for the first time a series of reversible organic
clathrates assembled in one-step that can incorporate a single or
a blend of PAHs into crystalline solids at RT is described. In
these solids, recognition pockets are built by the cooperative
assembly based on the combination of an arylboronic acid, a
diol (or the corresponding boronate ester 1), and a suitable
diamine linker into N→B adducts (i.e., A1), which are further
linked through CH···π interactions forming an octahedral-
shaped cavity. A strategy for the reversible “catch and release”
of PAHs under mild conditions has been projected. We
envision that a combination of other di- or oligoamine
connectors and suitable arylboronic esters might provide the
required complementary sites toward heavier PAHs as well as
other aromatic heterocycles.
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ASSOCIATED CONTENT
* Supporting Information
Synthetic procedures and spectroscopic analyses, TGA, PXRD
and HPLC data. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
(8) (a) Luisier, N.; Schenk, K.; Severin, K. Chem. Commun. 2014, 50,
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Sereda, O.; Neels, A.; Severin, K. Angew. Chem., Int. Ed. 2011, 50,
3034.
S
(9) For selected examples of purely organic clathrates as host−guest
systems see: (a) Lee, J. J.; Sobolev, A. N.; Turner, M. J.; Fuller, R. O.;
Iversen, B. B.; Koutsantonis, G. A.; Spackman, M. A. Cryst. Growth Des.
2014, 14, 1296. (b) Natarajan, P.; Bajpai, A.; Venugopalan, P.;
Moorthy, J. N. Cryst. Growth Des. 2012, 12, 6134. (c) le Roex, T.;
Nassimbeni, L. R.; Weber, E. Chem. Comm. 2007, 1124. (d) Curtis, E.;
Nassimbeni, L. R.; Su, H.; Taljaard, J. H. Cryst. Growth Des. 2006, 6,
2716.
AUTHOR INFORMATION
Corresponding Author
*Fax: (+52) 77-73-29-79-97. E-mail: hugom@uaem.mx.
Notes
■
The authors declare no competing financial interest.
(10) Hopfl, H. J. Organomet. Chem. 1999, 581, 129.
̈
ACKNOWLEDGMENTS
■
(11) HPLC = high performance liquid chromatography. Because of
the limited solubility and dynamics of these adducts in solution, it was
not possible to accurately measure the composition by NMR
spectroscopy, but the 1:1 composition was confirmed by elemental
analysis for ANT@A1 (see SI).
This work was supported by Consejo Nacional de Ciencia y
́
Tecnologia (CONACyT) through Project No. CB2009-
132227. The authors gratefully acknowledge for awarding
access to the instrumental facilities in the Chemical Research
Center at the Autonomous State Univeristy of Morelos (CIQ-
UAEM).
(12) Crystal data for [(acetone)2@A1] (CCDC 1040427);
C44H32B2N2O4·(C3H6O)2; Mr = 790.49, 0.553 × 0.333 × 0.188
mm3, triclinic, space group P1, a = 9.0771(3) Å, b = 10.8773(3) Å, c =
̅
11.3414(4) Å, a = 77.9240(10)°, b = 81.0780(10)°, g = 76.8330(10)°,
V = 1059.41(6) Å3, Z = 1, rcalc=1.239, T = 298 K, 2Θrange = 1.955 to
25.404°. 12558 reflections measured, 3867 unique (Rint = 0.0312), R1
= 0.0446 for 3191 reflections with I > 2σ(I) and wR2 = 0.1231 for all
data, 309 parameters, GOF = 1.036. (b) Crystal data for [ANT@A1]
(CCDC 1040428); C44H32B2N2O4·(C14H10); Mr = 852.55, 0.455 ×
0.148 × 0.097 mm3, monoclinic, space group P21/c, a = 7.3455(3) Å,
b = 20.4878(9) Å, c = 14.8292(7) Å, a = 90°, b = 103.6060(10)°, g =
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